The present invention relates to a method for converting a hydrogen fluoride alkylation unit to a sulfuric acid alkylation unit with a minimal amount of modification.
Sulfuric acid (H.sub.2 SO.sub.4) and hydrogen fluoride (HF) have competed for many years for choice as the preferred alkylation catalyst for converting isobutane and light olefins to gasoline. Alkylation units adapted to use sulfuric acid as a reaction catalyst and alkylation units adapted to use HF as a reaction catalyst are currently in use. Both HF and sulfuric acid are hazardous materials. However, at ambient conditions, HF is a toxic, volatile gas while sulfuric acid is a toxic liquid, making sulfuric acid much easier to contain in the event of an accident. Refiners and citizens living near refineries are becoming more aware and concerned about the toxic and volatile nature of HF. Although extraordinary safety procedures are currently incorporated into HF alkylation units, recent incidents and testing show that HF aerosol releases past refinery boundaries are possible. Because of growing concerns over the hazardous nature of HF alkylation units, legislation has recently been approved which will effectively ban HF alkylation in certain parts of the United States.
In response to the growing concerns over HF alkylation, several refiners have added mitigation equipment to their HF alkylation units to minimize potential releases of HF outside the units. The primary mitigation equipment includes water sprays to "knock down" the HF aerosol before it leaves the refinery. Due to the large quantity of water needed to mitigate an HF release, such a mitigation system is costly. The installation of HF release mitigation systems may create a false sense of security. The accepted maximum effectiveness of HF release mitigation equipment is only 90%. The possibility of HF escaping the mitigation system is a real possibility. In addition, the HF release mitigation equipment installed in a refinery cannot protect against an HF release during unloading operations or from a rupture of trucks or railroad tank cars loaded with HF during transportation of HF to the refinery.
Due to the hazardous nature of HF, it would be desirable to be able to convert an alkylation unit adapted to use HF as the reaction catalyst to an alkylation unit adapted to use sulfuric acid as the reaction catalyst. However, due to differences in the properties of HF and sulfuric acid, as well as differences in HF and sulfuric acid alkylation reaction chemistry, sulfuric acid cannot simply be substituted for HF in an HF alkylation unit.